Traditional commercial aircraft systems typically employ a constant frequency alternating current (ac) power distribution network in which various electrical motors may be coupled to an ac bus. Unfortunately, where such devices are directly coupled to the ac bus it is common to have a large inrush current present at equipment start up.
As commonly found in such a system, the variable speed of the main engine is converted into a constant frequency output via a mechanical interface device positioned between the main engine and various accessories, such as an ac electric generator or one or more auxiliary power units and lube pumps. A shaft running at a constant speed at the output of this mechanical device is used to rotate the accessories, including the main engine generator, thereby providing a constant frequency ac bus. However as advancements in aircraft system technologies become reality, it is contemplated that the constant frequency ac bus could be abandoned to allow the elimination of the mechanical interface between the main engine and the generator described above. Such advancements could further allow a system which would directly couple the ac electric generator to the main engine output shaft via a gearbox device.
The result of such direct coupling is an ac bus frequency value that would be proportional to the engine speed, and where the magnitude of the ac bus voltage would be regulated to a constant value via a generator control unit (GCU) for the generation system. The sizing of the accessory motors, such as those used for pump and fan applications for constant voltage-variable frequency systems, becomes significantly higher if each are directly connected to the constant voltage-variable frequency distribution system. Similarly, the inrush current requirements based upon these devices can also become substantial such as occurring in traditional commercial aircraft power systems.
For all these reasons, it is not feasible to directly couple these electrical machines directly to the constant voltage-variable frequency ac bus. However these machines can be connected to the bus via a rectification and inverter system such as a simple 6-pulse bridge rectifier. Unfortunately, due to the nonlinear nature of the operation of the 6-pulse rectification scheme, the input current drawn from the ac distribution system can become quite distorted. The frequencies at which characteristic harmonics are produced by such an input rectifier can be formulated as noted in equation (1) below.fH=(k×q±1)×f1  (1)In equation (1), fH=the characteristic harmonic, H=the number of harmonics, k=an integer beginning with 1, q=an integer representing the number of commutations per cycle, and f1=the fundamental frequency.
The characteristic harmonics of a system such as a 6-pulse rectification system including 5th, 7th, 11th, 13th, 17th and 19th harmonics, can have considerable magnitudes. Therefore the total harmonics distortion (THD) of current can become quite high and in some applications, can exceed 40% of the fundamental current value. These harmonics are not desired in the distribution system for numerous reasons and can easily exceed the specification requirements. Such undesired harmonics can result in increased power losses and thereby require an increase of the kVA rating of both the generation and distribution systems. Additionally, the current harmonics can distort the voltage waveform at a point of regulation. For these reasons, manufacturers require substantial reduction in these harmonic current values.
Accordingly, a need exists for an autotransformer system and method that allows the connection of electrical machines to an ac bus while providing the required substantial reduction in the harmonic values described above.